Biyolojik aşırı fosfor gideren aerobik granüler biyokütlenin mikrobiyolojik özellikleri

Özet

Tam-ölçekli konvansiyonel bir biyolojik arıtma tesisinden alınan floküler biyokütle ile başlatılan laboratuvar-ölçekli ardışık kesikli reaktörde, anaerobik/aerobik işletme ve fosfor ile besleme sonucu biyolojik aşırı fosfor giderimi (BAFG) elde edilmiştir. Çökelme süresinin kısaltılması (15 dak), ilk hacmin düşürülmesi (V0 1.8 L), hacimsel karbon yüklemesinin yükseltilmesi (1.41 kg KOİ/m3.gün) ve havalandırma kaynaklı kesme kuvvetinin artırılması (0.19 cm/s) ile, fosfor depolayan organizmalar (PAO) gibi yavaş-büyüyen organizmaların varlığı ile iyileşeceği öngörülen aerobik granülasyon süreci desteklenmiştir. Üstün çökelme özelliklerine (ÇHİ< 40-50 mL/g) sahip aerobik granüler BAFG biyokütlesi ile kararlı halde %92 karbon, %99 fosfor ve %78 azot giderimi elde edilmiştir. Sistemin biyokimyasal performansının izlenmesine paralel olarak, aerobik granüler BAFG biyokütlesinin mikrobiyolojik değerlendirmesi için morfolojik ve ekofizyolojik incelemeler gerçekleştirilmiştir. Hücre-içi poli-P (poli-fosfor) ve PHB (poli-hidroksi-bütirat) depolarının görsel tespiti için uygulanan Neisser ve Sudan Black B boyamaları sonucu, biyokütlenin morfolojik ve ekofizyolojik açılardan çeşitlilik gösterdiği saptanmıştır. Sistemde baskın tür, tanımlanmış morfolojileri ve ekofizyolojileri ile çubuksu PAO’lardır. Bunların yanısıra, morfolojik olarak glikojen depolayan organizmalara (GAO) benzeyen ancak ekofizyolojik özellikler açısından GAO-fenotipine uymayan tetrad/sarcina-benzeri hücreler (TFO) belirlenmiştir. Ayrıca, diplo-kokkoidlere, yoğun kokoid topluluklara, az miktarda filamentlere ve çeşitli protozoalara rastlanmıştır. Mikroskopik gözlemler niteliksel olmakla birlikte, sistemin biyokimyasal dönüşüm süreçleri bağlamındaki niceliksel performansı ile örtüşmektedir. Burada mikrobiyolojik özellikleri özetlenen aerobik granüler BAFG biyokütlesinin, mühendislik uygulamaları bağlamındaki üstün özellikleri nedeniyle, bu uygulamanın biyolojik atıksu arıtımında yeni ve gelecek vaadeden bir seçenek olacağı öngörülmektedir.

Abstract

A lab-scale sequencing batch reactor (SBR) was inoculated with a floccular biomass obtained from a conventional full-scale biological wastewater treatment plant. Enhanced Biological Phosphorus Removal (EBPR) was obtained through application of a sequential anaerobic/aerobic operational mode together with the metabolic selection strategies of anaerobic feeding (with acetate as sole C-source) and supply of phosphate. The first strategy was applied to maintain absolute elimination of the feast period, where direct and fast growth on acetate was possible together with simultaneous C-storage; the former promoting growth and dominance of fastgrowers having a negative impact on aerobic granulation. The merit of the anaerobic-feeding strategy was the possibility of directing the entire flux of externally available C-source to anaerobic C-storage mechanism, thus selecting the micro-organisms with the metabolic capability of taking up the acetate under anaerobic conditions, converting it to intracellular C-storage products (i.e., PHB: poly-hydroxybutyrate), and then growing slowly on these storage materials at the aerobic phase; the metabolic processes described for the PAO (Phosphate Accumulating Organisms) and GAO (Glycogen Accumulating Organisms) phenotypes. The second strategy was applied to promote dominance of the PAOs in the system. Aerobic granulation process, suggested to be enhanced by the presence and dominance of slowlygrowing microorganisms (like PAOs), was supported via lowering operationally determined settling time and initial reactor volume (TS decreased from 30 to 15 min and V0 decreased from 2.9 to 1.8 L), increasing volumetric COD load (from 0.24 to 1.41 kg/m3.d), and slightly increasing the shear rate due to aeration ($V_{SAir}$ increased from 0.14 to 0.19 cm/s). Monitoring the long-term steady state system performance in terms of biochemical conversion processes indicated that it was possible to secure high carbon, nitrogen and phosphorus removal efficiencies (92% COD removal, 99% EBPR, 78% overall N-removal) with the aerobic granular biomass upholding superior settling properties (SVI< 40-50 mL/g). Parallel to the evaluations with regard to biochemical system performance, morphological and ecophysiological examinations via conventional microscopy and chemical staining techniques were also executed to determine the microbiological features of the aerobic granular EBPR biomass, and to help confirm and interpret system performance with respect to presence of different microbial groups. Application of Neisser’s and Sudan Black B stains to the biomass samples for visualization of intracellular volutin poly-P (poly-phosphate) granules and lipophilic PHB inclusions, respectively, revealed a microbial community rich in terms of morphological and eco-physiological traits. The dominant phenotype in the system was the PAOs with their conventional rod-shaped morphology and typical EBPR-physiology of being strongly poly-P(-) and strongly PHB(+) at the end of anaerobic phase, whereas being mostly poly-P(+) and partly PHB(-) at the end of the aerobic-period. In addition to the PAOs, tetrad/ sarcina-like cells resembling the GAOs in terms of morphological features were also of significance. However, phenotypic properties of these microorganisms were not in line with those of the GAO-phenotype. Thus, the tetrad/sarcina-like cells, which were PHB(-) both at the end of the anaerobic- and aerobic-phases, were named as “TFOs” (Tetrad Forming organisms), a term for morphological differentiation, rather than “GAOs”, a term related with functional properties. The PAOs and the TFOs co-existed with some other morphotypes like diplococci- shaped cells, staphylococci-like clustered populations, and a few filaments with an abundancy of 0-1 in accordance with subjective-scoring. Presence of some fixed protozoa like Vorticella campanula and Carchesium spp., typical for activated sludge systems, was also recorded. Despite the microscopic observations were qualitative, they correlated well with the quantitative biochemical performance data. Finally and from an engineering stand-point, operational flexibility of the SBR configuration, superior settling properties of granular biomass thus possibility of working with smaller reaction volumes or with shorter reaction times, and possibility of decreasing aeration related costs due to presence of an anaerobic phase, together demarcate the Aerobic Granular EBPR Technology as a promising biological wastewater treatment alternative enabling to decrease capital and operational costs while securing desired removal efficiencies.